The invention is related to the fields of broadband cable television systems and is most closely related to laser optical communication links for such systems.
In a cable television system, television programs are provided at a central head-end. The programs are distributed from the head-end through optical fiber tree networks to multiple local nodes in respective communities, and then further distributed from the local nods through coaxial cable tree networks to customer interface units (CIUs) also called cable terminations. Currently, many of these systems are beginning to provide other communication services such as telephone service and/or computer networking services (e.g. internet connection) through the cable television system. Telephone and computer networking services require bi-directional communication in the cable television system. Forward communication signals are transmitted, as described above for television program signals from the head-end to the customer interface units, and return communication signals travel the same path in the opposite direction. The return signals are collected from the CIUs through the coaxial cable networks to the local nodes, and further collected from the local nodes through the optical fiber network to the head-end.
At the head-end, a multitude of electronic signals for the programs and other communication services are used to modulate respective carrier signals with different respective frequencies. The modulated carrier signals are combined together into an electronic forward signal that is used to modulate a laser beam to produce an optical forward signal. The modulated laser beam, carrying the optical forward signal, is transmitted through an optical fiber tree network to a multitude of local nodes. At each local node, an optical detector coverts the optical forward signal back into an electronic forward signal. Then the reconverted electronic forward signal is transmitted through a coaxial conductor tree network to CIUs at homes and businesses of customers.
Telephone systems and computer systems connected to the CIUs by customers, produce return communication signals that are transmitted by the CIUs into the coaxial network. The return signals are multi-carrier modulated signals similar to the forward signals. The return signals travel back through the coaxial network to the local nodes. In the local nodes the return signals are separated from the forward signals by diplex filters. The separated return signals are used to modulate a return laser beam to produce an optical return signal carried by the return laser beam. The optical return signal is transmitted through an optical fiber network to the head-end where the optical return signals are converted back into electronic return signals by an optical detector. The electronic return signals are demodulated and used for telephone and computer communications.
Laser diodes are used to produce the laser beams that are modulated to convert the electronic signals into optical signals at the head-end and at the local nodes. In a directly modulated laser diode, the intensity of the laser beam depends on the current applied to the laser diode. The laser produces a signal as long as the current through the diode is positive and above a cutoff current level for the diode. Below the cutoff current level, the intensity of the laser is non-linear and falls quickly to zero. The current through the laser diode is modulated so that a modulation signal is carried by the laser beam. In order to produce a continuous signal, that is not cut off every time the signal becomes negative, the modulation signal is biased (e.g. a bias current is modulated by the modulation signal) so that the intensity of the laser beam produced by the laser is continuously modulated and negative portions of the signal are not lost. The electronic information signal includes positive and negative excursions of amplitude and the extent of some of the excursions are larger than other excursions. The bias is set so that the minimum amplitude of the biased electronic signal during the largest negative excursions of the signal is equal or higher than the cutoff bias of the laser diode.
The modulation index is the ratio between the power of the modulation of the laser beam and the total power of the laser beam. Thus, the modulation index is a measure of the energy efficiency of the communication so that increasing the modulation index reduces the energy required for the optical communications. In addition it has been found that the signal to noise ratio (SNR) is approximately proportional to the modulation index.
In order to transmit information without loss, it is critical to maximize the SNR. There are strict specifications for minimum SNR for all types of communications equipment, and the SNR requirements limit the distance through which signals may be transmitted through optical fiber links and coaxial cable links to customers. Generally, the noise in each stage of the communication system is additive to reduce SNR.
Those skilled in the art are directed to U.S. Pat. No. 4,941,208 to Olshansky in which a multitude of signals modulated by carriers of different frequencies are combined into a multi-carrier signal in which the sum of the modulation indexes of the signals is greater than one.
The above references are hereby incorporated herein in whole by reference.
In the invention herein, at a first node, an output electronic information signal is used to modulate a laser beam resulting in an optical information signal that is transmitted through an optical fiber to another node where, an optical detector converts the optical information signal into an input electronic information signal. The output electronic information signal includes high frequency positive and negative excursions of with respect to an average amplitude of the signal, and the extent of some of the excursions are larger than other excursions.
Prior to modulating the laser beam, the output signal is preprocessed to improve the resulting optical information signal. The preprocessor includes pre-shaper that transforms the electronic information output signal to reduce the extent of the larger negative excursions, so that, the modulation index can be increased for increasing the signal to noise ratio and increasing the energy efficiency of the communications. The transformed output signal is biased so that the current level in the biased signal is generally above a predetermined current level (e.g. the cut off current level of a laser), and the biased transformed signal is used to modulate the laser beam to provide an optical information signal.
Preferably, the output signal is a multi-carrier signal including a multitude of carrier signals each of a different frequency and each modulated by a respective baseband information signal.
The transformation may be a simple clipping of large negative excursions which provided continuous output of the laser. Clipping of large positive excursions may provide reduced noise if the signal with such positive excursions that are distorted are more noisy than the clipped and approximately restored signals. If post shaping is provided, cut off peaks can be estimated based, for example, typical shapes of cut off excursions based on the width of the clipped portion and/or on the derivative of the signals to the beginning and end of the cut off portion. More preferably, the transformation is a function selected to minimize third order distortions due to using the output signal to modulate a laser beam and due to transmitting the modulated laser beam through an optic fiber. Preferably, the transfer function is a parabolic transfer function. Also, the modulation index can be further increased by selecting a transfer function which reduces the extent of larger positive excursions with respect to the other excursions in the output signal so as to further increase the modulation index and further reduce distortion and noise resulting from modulating the laser beam.
Preferably, parameters of the transformation are statically adjusted at the factory or during installation of the circuit or are dynamically modified during operation depending on parameters of the optical output signal either manually from a front panel or automatically depending on a feedback, for example, from a receiver that receives the output signal.
Transforming the signal induces second order distortion of the output signal during transmission through an optic fiber, so preferably, the frequencies of carriers for critical signals in the output signal are within a range of one octave, from a minimum frequency of f1 to a maximum frequency of f2 where f2 less than 2*f1, so that second order distortion can be filtered out after the optical fiber transmission. Also, the transforming induces fourth order distortion of the output signal during transmission through an optical fiber, so preferably, the frequencies of carriers for critical signals in the output signal are within a range of half an octave, from a minimum frequency of f1 to a maximum of frequency of f2 where f2 less than 1.5*f1, so that fourth order distortion can be filtered out after the optical fiber transmission. Also, preferably, the carrier frequencies of critical signals are between approximately 550 and 750 MHz in a CATV network.
Preferably, the preprocessor also includes a pre-compensating circuit for compensating for distortions in the communication system. The pre-compensating circuit should be an in-line compensating circuit so that high frequency signals can be processed. The pre-compensating circuit distorts the output signal to compensate for odd order distortions due to dispersion when transmitting the laser beam through an optical fiber. The pre-compensating circuit also distorts the output signal for compensating for odd order distortions due using the output signal to modulate a laser beam.
Even when second and fourth order distortions are filtered out, preferably, the pre-compensator circuit also compensates for even order distortions especially sixth and higher even order distortions. These even order distortions may be due to using the output signal to modulate the laser beam and due to dispersion when transmitting the modulated laser beam through the optical fiber. The pre-compensator circuit also compensates for distortions due to receiving the output signal from the modulated laser beam with a photo-detector and due to amplifying the information signal. For example, the optical output signal can be amplified using an dope fiber optical amplifier and the output and input electronic signals can be amplified using preamplifiers and power amplifiers.
A optical transmitter of the invention is defined by the pre-processor together with, a biaser to bias the output signal so that the minimum amplitude is higher than a predetermined minimum positive amplitude of the output signal current, a laser to produce a laser beam, apparatus for modulating the laser beam with the output signal, and apparatus for directing the laser beam into the end of an optical.
Preferably, the laser is a directly modulated laser so that the laser and the means for modulating the laser beam are integral, and the predetermined minimum positive amplitude corresponds approximately with the cutoff amplitude of the directly modulated laser. The inventions described in this application are especially useful for distributed feedback type laser diode. Such a laser can be directly modulated by the bias current.
Preferably, a multitude of signal inputs are provided for respective baseband signals and the baseband signals are modulated and combined to form a output signal. Modulators are used for modulating respective carrier signals with each baseband signal. The frequencies of the carrier signals are different so that the modulated carrier signals can be combined and then separated using a tuner. A combiner may be used for combining the multitude of carrier signals from different respective conductors into a output signal in a single conductor.
Preferably, the bias level provided by the biaser is adjusted at the factory or manually during installation or more preferably is adjusted dynamically during operation depending on parameters of the optical output signal, either manually from the front panel or automatically depending on a feedback for example from a receiver that receives the optical signal.
Preferably, the transmitter includes an amplifier for amplifying the output signal prior to using the output signal for modulating the laser beam, and the transmitter includes an optical lens system through which the laser beam travels between the laser and the proximate end of the optical fiber.
When the optical signal reaches the other node it is converted into an input electronic signal. In the other node, the input signal is provided to a signal post-processor that includes a post-shaper for reforming the input signal to approximately duplicate the output signal prior to transforming the output signal to increase the modulation index, modulating a laser beam with the transformed signal, transmitting the modulated laser beam through an optical fiber, and converting the laser beam into the electronic input signal. The input signal is a high frequency electronic signal with positive and negative excursions in amplitude with respect to an average amplitude with the extent of some excursions being larger than other excursions. The reforming includes increasing the extent of larger negative excursions with respect to other excursions. The input signal is then provided to the other node through an output for the reformed signal.
Preferably, the input signal is a multi-carrier signal including a multitude of carrier signals each of a different frequency and each modulated by a respective baseband information signal. The post-shaper is also adapted for increasing the extent of larger positive excursion with respect to the other excursions in the input signal. In the case were such positive excursions were reduced prior to transmitting this provides better duplication of the original output signal to its prior to reducing the extent of the larger positive excursions.
Preferably, the post-processor also includes post-compensator apparatus to compensate for distortions in the input signal. The post-compensator apparatus includes a filtering circuit for filtering out second order distortions from the input signal when critical carrier frequencies in the input signal are limited to a range of one octave. The filtering circuit may also be adapted for filtering out fourth order distortions from the input signal when the carrier frequencies in the input signal are limited to a range of half an octave.
Preferably, the post-compensator apparatus includes a linearizing circuit to distort the input signal to compensate for distortions in the input signal. The linearizing circuit is preferably an in-line linearizing circuit so that it can operate at very high frequencies. The linearizing circuit may include apparatus for removing odd order distortions due to modulating laser beam with the input signal and for removing odd order distortions due to receiving the input signal from the laser beam with the photo-detector, and amplifying the input signal such as a preamplifier and power amplifier in the receiver. The linearizing circuit may also include apparatus for removing at least part of the odd order distortion due to transmitting the laser beam through optical fiber. This may be necessary in a system such as a CATV system, in which the same signal is sent to multiple nodes at different distances so that part of the distortion due to transmission through the fiber varies between receiving nodes.
An optical receiver of the invention is defined by the post-processor together with an optical detector for converting the input laser beam into an electronic input signal and apparatus for directing the input laser beam from the optical fiber onto the optical detector. The post-processed input signal is directed into the other node through an output of the receiver.
Preferably, the optical detector is based on a PIN photo-diode, and the input optical signal is directed onto the photo-diode an optical lens system through which the laser beam travels between an optical fiber and the optical detector. The receiver includes a preamplifier after the optical detector which is followed by the post-shaper and a post-compensator. A power amplifier is preferably positioned after the post-shaper and post-compensator to minimize the power dissipated by those components.
The apparatus for directing the laser beam onto the optical detector may be, another lens system or possibly a direct connection, and the optical detector may be a PIN photo-diode. The detector converts the optical information signal carried by the laser beam into an input electronic information signal.